The present invention relates to a method for detecting impurities in transparent material whereby the material is scanned line by line and the light transmission through the material is measured and compared with a reference value, and also to a system for detecting impurities in a transparent material, which system comprises a camera arranged to scan the material and register light transmitted through the material in the form of pixel values, and a first comparator for comparing said pixel values with a reference value.
The present invention relates primarily to detecting various types of impurities in transparent polyethylene or polypropylene materials for various uses, such as the production of pipes, cable casing, thin films, etc.
The starting material used in the production of such polyethylene or polypropylene materials is in the form of pellets which are heated to a molten mass and compressed to form a tape or film. During this procedure clumps of molecules may be formed having a different structure from the surrounding material. The reason for these clumps of molecules, named gels in this context, may be poor melting of the starting material or deficiencies in earlier polymerisation steps for producing the starting material. The gels thus constitute a kind of lack of homogeneity in the material produced, in the form of collections or clumps of molecules with a different, incorrect structure from the surrounding flawless material. These gels are at least as hard as the surrounding material and may even be harder. The accepted definition of a gel as something that has coagulated to an elastic, semi-solid mass thus does not coincide with the clumps of molecules that are termed gels in the present context.
Gels have special optical properties. They function as lenses, and this property is utilized in the technology known hitherto for gel detection. The optical properties of the gel thus deviate from those of the surrounding material and gels have hitherto been detected using equipment comprising a light source and a light detector, the optical axes of which are displaced so that the detector does not normally see the point illuminated by the light source and only does so when something changes the light deflection, i.e. a gel appears so that the light is deflected into the detector, see U.S. Pat. No. 4,492,477 and WO 97/43624, for instance. The drawback of this previously known technology is that it does not permit simultaneous detection and separation of gels and other types of impurities.
Gels may be of extremely varying sizes and have different shapes, usually annular even if only a circle segment is detectable. Common to the gels, however, is that they have a lighter central part, i.e. a central part with higher light transparency than surrounding parts, as compared with other types of impurities that are usually darkest at or in the vicinity of the centre. This difference in transparency between the various types of impurities is utilised in the present invention in order to alleviate the drawbacks with the previously known technology.
The object of the present invention is thus to provide a method and a system for detecting different types of impurities in a transparent material and to separate the impurities.
This object is achieved with a method and a system of the type described in the introduction, with the characterizing features defined in claims 1 and 6, respectively.
The present invention is thus based on pattern recognition. As mentioned above, gels and other types of impurities produce different light transmission patterns upon illumination and according to the invention pattern recognition technology is utilised not only to detect different types of impurities but also to separate them. According to the present invention, thus, xe2x80x9careas of interestxe2x80x9d are detected first by comparing the light transmission with a reference value, whereupon this area is then analysed in more detail in order to determine the impurity. In this way a great majority, up to 99%, of the light transmission data can be sorted out since areas of interest appear rather seldom. This reduction or compression of data facilitates the subsequent analysis of light transmission values measured so that, even if the light transmission data from such xe2x80x9careas of interestxe2x80x9d arrive in showers, these showers of data can be processed in the following in a relatively simple manner.
According to advantageous embodiments of the method according to the invention light transmission values measured are compared with the reference value of the light transmission in order to determine the extent and shape of the impurity. The centre of the impurity is determined on the basis of its extent and shape. Higher light transmissions measured in the centre of the impurity than in the surrounding parts are indicated as an impurity of gel type. As mentioned, the gels are often circular even if only a circle segment is detectable.
According to an advantageous embodiment of the system according to the invention, a reference-producing unit is arranged to produce the reference value from the value or values of one or more selected pixels measured previously. By utilising a floating reference value in this way instead of a fixed reference value as in previously known technology, the sensitivity of the system is increased for detecting small changes in the material being scanned, and by also choosing a value from a greater or smaller number of previously measured pixel values as reference value, the sensitivity of the detection can be controlled.
According to another advantageous embodiment of the system according to the invention, the reference-producing unit is arranged to produce a reference value from the mean value of the values of a predetermined number of pixels immediately prior to the pixel being currently scanned. The relevant pixel is thus compared with a reference deriving from recent previous pixel values, which makes this embodiment suitable for detecting rapid changes in the material being monitored or scanned.
According to other advantageous embodiments of the system according to the invention, wherein the camera is a linear camera for scanning the material line by line, scanned lines are stored line by line and the reference-producing unit is arranged to choose as reference value the value of the pixel in a selected stored line corresponding to the relevant pixel in the line being scanned. These stored reference lines provide a sort of historical record containing information as to how the appearance of the scanned material has varied previously. In this embodiment of the system according to the invention, changes in the scanned material are determined over a period of time that can be chosen through the choice of reference line. By choosing a xe2x80x9crecentlyxe2x80x9d stored reference line immediately before the new, incoming line, the change is measured over a short time interval. This is advantageous if rapid changes are to be detected. To detect slower changes, the line being scanned is compared with a reference line stored longer ago, i.e. a comparison in time is made over shorter or longer periods depending on the requirement of the measurement in progress. By choosing the time over which changes shall be detected, thus, the sensitivity of detection can be adapted to current requirements. The time shift between the stored lines in turn corresponds to a certain spatial displacement in the case of movable material.